In these years, in disk apparatuses such as hard disk apparatuses, the density of data stored on a magnetic disk tends toward being made higher. Accordingly the track pitch of the magnetic disk tends toward becoming narrower. When reading out data from a magnetic disk of a narrower track pitch by a magnetic head, if an off-track occurs in a track width direction, data written on the track may not be accurately read out. Or the occurrence of an off-track in a track width direction, when writing data onto a magnetic disk of a narrower track pitch by a magnetic head, results in the possibility that the data written onto the track may not be accurately read out when reading out the data by the magnetic head later.